nai

Project Progress

Significant debate exists as to whether early forms of life on Earth were autotrophic or heterotrophic. This debate extends to early earth analog hot spring environments, where most scientists assume chemotrophic communities to be largely supported by autotrophic metabolisms. However, evidence to support this claim is minimal. To further evaluate the carbon metabolism of chemotrophic communities we undertook a study aimed at determining the preference for autotrophic or heterotrophic metabolism in populations inhabiting high temperature hot springs. Formate and acetate were selected to represent simple forms of organic carbon that could potentially be generated on an early Earth via abiotic water-rock interactions and which could be readily integrated into the metabolisms of early heterotrophs. Rates of formate and acetate assimilation were compared to rates of dissolved inorganic carbon assimilation (DIC) in 13 high temperature (>73°C) hot springs in Yellowstone National Park (YNP). While 9 of the hot spring communities exhibited rates of DIC assimilation that were greater than that of formate and acetate, 2 exhibited rates of formate and/or acetate assimilation that exceeded that of DIC assimilation providing evidence that not all chemosynthetic communities are supported by autotrophic carbon metabolisms. Overall rates of DIC, formate, and acetate transformation were positively correlated with spring pH but showed little correlation with temperature. Communities sampled from hot springs with similar geochemistry generally exhibited similar rates of substrate transformation. This is consistent with similar community compositions in springs with similar geochemistry as revealed by 16S rRNA gene tagged sequencing. Amendment of microcosms with low (µM) amounts of formate suppressed DIC assimilation and the concentration of added formate required to suppress DIC assimilation was similar to the affinity constant (Km) for formate transformation as determined by community kinetic assays. These results suggest that dominant chemoautotrophs in high temperature communities are facultatively autotrophic, adapted to fluctuating nutrient availabilities, and are capable of taking advantage of energy-rich organic substrates when they become available.

Chemoautotrophic life is sustained by inorganic sources of electron donors and acceptors. Previous studies have suggested a central role for hydrogen metabolism in supporting high temperature hot spring communities. To further evaluate this hypothesis we characterized rates of hydrogen transformation in eleven chemosynthetic communities inhabiting hot springs that were located in regions of Yellowstone with differing subsurface bedrock. In addition, we quantified rates of autotrophic and heterotrophic metabolism (formate and acetate), and collected samples for RNA-based characterization. To provide context for the community composition and metabolic activity measurements, we determined in situ concentrations of a suite of potential microbial substrates including hydrogen, carbon monoxide, low molecular weight organic acids, as well as a range of electron acceptors. These measurements provide a basis for comparing (a) the substrate-specific kinetic (enzyme uptake) data against naturally occurring concentrations of those same substrates; and (b) the landscape of energy availability in various metabolic processes, determined as both molar Gibbs energy change and as metabolic power generation potential, against the observed distribution of metabolic activity across a range of physical and chemical variables. Preliminary results indicate substantial rates of hydrogen transformation that are correlated with high rates of primary production. These findings are consistent with the conclusions of the aforementioned study indicating that hot spring communities are supported primarily by carbon introduced through autotrophic metabolism and this metabolism may be fueled by hydrogen.